Frustration with the pace of the negotiations with Iran is understandable, as are fears that delaying tactics are buying Iran time to develop a nuclear weapon.

However, a congressional vote threatening Iran with more sanctions if an agreement is not reached on Congress' timetable could prove counterproductive, giving Iran an excuse to leave the negotiation table and place the blame on the United States for the breakdown in talks. That in turn could make it more difficult for the U.S. to maintain the international support for sanctions that is necessary to apply sufficient pressure on Iran.

Current negotiation terms call for all sides to reach a political framework by March 24 that would set the stage for a final agreement by June 30. The United States and its bargaining partners - Britain, Russia, China, France and Germany - seek assurances that Iran has abandoned its quest to produce nuclear weapons and agreed to international monitoring to verify its compliance.

The talks date back to a tentative agreement reached in November 2013.

In a letter to the president on Tuesday, eight fellow Democrats in the Senate, including Connecticut Sen. Richard Blumenthal, essentially gave President Obama a deadline. They said they were willing to wait until March 24 to see if a political framework agreement is indeed reached with Iran.

If that deadline passes, write the senators, they are ready to join with Republicans in approving the Iran Nuclear Weapon Free Act of 2015, which would impose sanctions on Iran if negotiators do not reach a comprehensive deal by June 30.

Sen. Blumenthal and the Senate should be giving the executive branch, which is closest to the talks, the right to assess when talks have reached an impasse and it is time to pull the plug on negotiations and impose more sanctions.

While talks continue, Iran has agreed to stop enriching uranium to levels necessary for developing weapons. Last year, the United Nations nuclear agency confirmed Iran, as agreed, had turned its existing enriched uranium into a more harmless form. And Iran has agreed not to install any new centrifuges.

If Congress must act, the better option is a bill supported by the state's other Democratic senator, Chris Murphy. It calls for new sanctions if the Iran talks fail, but does not handcuff the administration, and the other five countries involved in the talks, with a deadline.

As we've said in this space before, no priority in the region is higher than stopping Iran from getting a nuclear weapon, preferably peacefully by achieving a final deal, but militarily if necessary.

The military option cannot be taken lightly. Things are volatile in the Middle East. How matters would play out after an attack on Iran's weapons-production infrastructure is hard to predict. It is quite possible an Iranian military response could target neighbors, expand the conflict, disrupt oil supplies and damage the global economy.

It is possible, perhaps even likely, that Tehran is playing games, stalling with the intention of making a race to develop a bomb after it walks away from talks. But the Obama administration and Secretary of State John Kerry deserve the chance to continue testing the waters to determine if the Iranians are serious.

It would be unfortunate if congressional impatience derailed that effort prematurely.